THERE鈥橲 a type of stem cell that does not trigger rejection, even in
transplants from one species to another, researchers have discovered. That means
stem cells from a donor could be transplanted into anyone else, for example to
regrow bone, replace cartilage or repair damaged hearts.
The finding comes as a surprise. 鈥淚t does go against our common understanding
of the immune system,鈥� says Annemarie Moseley, chief executive of Osiris
Therapeutics in Baltimore, a company working on the 鈥渕esenchymal stem cells鈥�
(MSCs).
Preventing the immune system attacking implanted stem cells is a massive
problem. Stem cells derived from cloned embryos would be genetically identical
to the donor, which is why companies like Advanced Cell Technology are working
on therapeutic cloning
(see 鈥淚mpossible to ape鈥�).
Others are trying to engineer stem cells that
lack the genes for the cell surface proteins the immune system recognises and
attacks.
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But all this work may be unnecessary. Studies at Osiris and the Dana Farber
Cancer Institute in Boston have shown that MSCs from the bone marrow of adults
do not carry the markers on their surface that lead to rejection. And that鈥檚
still true even after the cells differentiate into specialised tissues such as
bone or fat, says Moseley.
In fact, the cells are so good at evading rejection that transplanting them
from pigs to rats does not trigger a response, according to unpublished work by
Ray Chiu of McGill University in Montreal.
Osiris has already given around 100 people MSCs donated by their family
members, some of whom were poor matches. Some of the cells were given to cancer
patients to replace bone marrow destroyed by treatment. Others are being used to
regenerate bone in the jaw. Up to four years later, says Moseley, the
transplants haven鈥檛 been rejected.
What鈥檚 more, Chiu has shown that MSCs injected into rats quickly home in on
the bone marrow. But within hours of a heart injury, the MSCs get into the
bloodstream, and can later be found in the rat鈥檚 hearts. The cells seem to go
only to damaged areas, Chiu says. 鈥淭hey turned into heart muscle, blood vessels
and fibrous tissue.鈥�
Osiris has found very similar results in pig studies. Clinical trials to test
the same approach in patients who have suffered heart injury are being worked
out with the FDA and are due to begin next year, says Moseley.
Research so far suggests that hospitals could keep pools of MSCs ready to
transplant into people who鈥檝e had heart attacks, for example, where they鈥檇 leap
into action to repair the damage. We already have reserves of our own MSCs, but
their numbers fall dramatically as we age.
Getting hold of the cells should be relatively easy. MSCs don鈥檛 grow
indefinitely in culture, but enough can be grown from a single bone marrow
donation to treat 10, 000 people or more.
If MSCs live up to their promise, there may be little need for embryonic stem
cells. But it鈥檚 not yet clear if they are as versatile. So far, MSCs have only
been shown to give rise to about six kinds of tissue, including bone, cartilage,
tendon and muscle. But mouse studies suggest their potential may be greater,
perhaps even including neural cells.
Some studies also show that MSCs do not turn into cancerous masses when
transplanted, as sometimes happens with embryonic stem cells. But even if MSCs
prove their worth in clinical trials, research with embryonic stem cells should
not be considered irrelevant, says Diane Krause of Yale University.
There might be many types of tissue that MSCs will not be able to repair,
says Krause. 鈥淚t would be wrong to conclude that research with embryonic stem
cells should stop.鈥�